Phonons at the zone center for α-NaAlH4

A short-range force constant model has been applied for the first time to investigate the phonons in α-NaAlH₄ having body centered tetragonal Scheelite structure .The normal symmetry coordinates for the Scheelite structure were computed to investigate the phonons at the zone center. The phonons for α-NaAlH₄ have been calculated involving five stretching and two bending force constants .The calculated Raman frequencies exhibit good agreement with the available measured values. The infrared frequencies have been assigned proper modes for the first time.     

Surface modifications of ultra-thin gold films by swift heavy ion irradiation

Gold films of thickness 10 and 20 nm grown on float glass substrate by thermal evaporation technique were irradiated with 107 MeV Ag⁸⁺ and 58 MeV Ni⁵⁺ ions at different fluences and characterized by Grazing Incidence X-ray Diffraction (GIXRD) and Atomic Force Microscopy (AFM). The pristine films were continuous and no island structures were found even at these small thicknesses. The surface roughness estimated from AFM data did not show either monotonic increase or decrease with ion fluences. Instead, it increased at low fluences and decreased at high fluences for 20 nm thick film. In the 10 nm film roughness first increased with ion fluence, then decreased and again increased at higher fluences. The pattern of variation, however, was identical for Ni and Ag beams. Both the beams led to the formation of cracks on the film surface at intermediate fluences. The observed ion-irradiation induced thickness dependent topographic modification is explained by the spatial confinement of the energy deposited by ions in the reduced dimension of the films.     

Sub-millimeter third harmonic generation in n-InSb waveguide

Resonant third harmonic generation of a sub-millimeter wave in n-InSb waveguide embedded with a density ripple is investigated. The non-linearity arises through the modulation of free electron mass while the ripple accounts for the phase mismatch. The efficiency of the third harmonic generation is large. However, as the plasma frequency increases the attenuation rate of the third harmonic increases and the third harmonic efficiency decreases as (ω_pb/c) is raised.     

Analysis of interface states and series resistance for Al/PVA:n-CdS nanocomposite metal–semiconductor and metal–insulator–semiconductor diode structures

This paper presents the fabrication and characterization of Al/PVA:n-CdS (MS) and Al/Al₂O₃/PVA:n-CdS (MIS) diode. The effects of interfacial insulator layer, interface states (N _ss ) and series resistance (R _s ) on the electrical characteristics of Al/PVA:n-CdS structures have been investigated using forward and reverse bias I–V, C–V, and G/w–V characteristics at room temperature. Al/PVA:n-CdS diode is fabricated with and without insulator Al₂O₃ layer to explain the effect of insulator layer on main electrical parameters. The values of the ideality factor (n), series resistance (R _s ) and barrier height (? _b ) are calculated from ln(I) vs. V plots, by the Cheung and Norde methods. The energy density distribution profile of the interface states is obtained from the forward bias I–V data by taking into account the bias dependence ideality factor (n(V)) and effective barrier height (? _e ) for MS and MIS diode. The N _ss values increase from mid-gap energy of CdS to the bottom of the conductance band edge for both MS and MIS diode.     

Laser-induced damage studies in GaAs

Laser-induced damage studies have been carried out on monocrystal GaAs at 1.06 μm wavelength as a function of pulse repetition rate in the nanosecond regime. The single shot observed laser damage threshold is 0.9 J/cm². It has been found that the damage threshold decreases when the sample is irradiated with large number of pulses. However, the above effect is observed only when the repetition rate is higher than 1 Hz. Various laser damage mechanism theories have been discussed to explain the results.     

K-resin based multilayer polymeric mode filter for integrated optics

We demonstrate theoretically as well as experimentally a new type of four layer polymeric waveguide structure which can be used as mode filter. Various optical properties such as refractive index, birefringence and propagation constant of SAN and K-resin are presented. The thin film structure consisting of glass/K-resin/SAN/air is used to act as mode filter. Expressions for the electric field intensity spatial distribution for the structure are used to calculate the intensity profiles to support the observed behavior. The experimental values were in good agreement with the one obtained theoretically.     

Optical properties of selenium-tellurium nanostructured thin film grown by thermal evaporation

Se-Te nanostructured thin films were deposited on glass substrates in the presence of oxygen and argon by thermal evaporation. The properties of Se-Te thin films strongly depend on the deposition method. During the process used, the substrate is cooled to a temperature of 77 K employing liquid nitrogen. The nanostructured thin films of Se_100−xTe_x (where x=4, 8 and 16) are deposited on glass substrate. The surface morphology of the deposited films was investigated through Scanning Electron Microscopy (SEM). The typical size of these nanostructures is in the range 40-100 nm and the length is of the order of several micrometers. The optical parameters i.e. optical gap (E_g), absorption coefficient (α), and extinction coefficient (k) are calculated in the wavelength range 190-1100 nm. It was found that the optical band gap decreased from 3.4 to 2.9 eV when Te concentration was increased in the Se_100−xTe_x nanostructured thin films. The large bandgap may be attributed to the decrease in particle size which clearly exhibits a quantum size effect. XRD analysis was performed to confirm glassy nature of the nanostructured thin films.     

Magnetic properties of undoped YBa 2 Cu 3 O 6 + x system

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon gap” has been attributed solely to the intra-bilayer exchange coupling (J _⊥ ) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T _N ) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T². Received 13 July 2000 and Received in final form 14 May 2001     

Electrical properties of a-Ge<Subscript>x</Subscript>Se<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>

In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of Ge_xSe_100-x in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of Ge_xSe_{100-x (x} = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (Ge_xSe_100-x ) have been successfully explained by correlated barrier hopping (CBH) model.     

Theoretical study of polymeric metal clad optical waveguide polarizer

Planar optical waveguides consisting of thin dielectric films and buffer layers with metal cladding have been investigated theoretically. A computer program was written to calculate the exact zeroes of complex eigenvalue equation for TE and TM modes in multilayer metal clad waveguide polarizer. Numerical results and illustrations are given for Polycarbonate waveguide with other polymers as buffer and Al, Ag and Au as cladding metals at . It is also shown that, using thin (finite) films of metal produce more efficient polarizers as compared to semi-infinite metal films. Effect of low index buffer layer on attenuation of TM/TE modes is also investigated.